- 學位論文
金、銀奈米桿之光學與力學特性研究
Optical Properties and Mechanical Responses of Gold and Silver Nanorods
摘要
本文主要研究為金、銀奈米桿之光學與力學特性，奈米桿因為幾何形狀上特殊，在表面電漿共振(surface plasmon resonance；SPR)模態中出現長軸表面電漿共振(LSPR)，此獨特現象使得金、銀奈米桿在光學特性與力學效應上有許多值得探討的地方，特別是近紅外線範圍。本研究依據Maxwell電磁理論作為基礎，並利用多重中心展開法作為模擬計算方法，分析含有金、銀奈米桿的電磁場，進而探討衍生的光學與力學特性。 研究主要分為兩部分，光學部分著重於模擬實驗中平面波入射金奈米桿水溶液的平均光學效應，包含散射、吸收與消散截面積平均效率和平均去極化效率，在截面積平均效率中亦討論簡化運算的三主軸入射計算法，結果顯示三主軸入射計算法只適用於描述SPR僅出現第一模態的細小金奈米桿。 力學部分本研究使用Maxwell應力張量來分析金、銀奈米桿上電磁場對其所產生之應力，將應力沿著奈米桿表面積分後取得光力，或乘上力臂再沿著表面積分後可得光力矩。在模擬分析結果發現金、銀奈米桿對線性偏極光所產生之光力矩具有兩種模態，分別為介於短軸SPR與LSPR範圍之間的垂直模態(perpendicular mode)，及波長長於LSPR的平行模態(parallel mode)，光力矩為零的時候我們稱之轉捩點(turning point)，轉捩點之波長正好為LSPR。此兩種模態的發現，可以清楚的解釋前人實驗的結果：對於同一波長的線性偏極光照射下，奈米桿因受到光力矩驅使，最終平行於入射波的電場極化方向，而奈米線則垂直於入射波的電場極化方向。這種造成奈米桿與奈米線有不同對齊(alignment)方向的原因為金、銀奈米桿隨著細長比的增加，該LSPR會隨之產生紅位移的現象。
並列摘要
In this thesis, we theoretically studied the optical properties and mechanical responses of gold (GNRs) and silver nanorods (SNRs). GNRs and SNRs have different kinds of polarizability: longitudinal surface plasmon resonance (LSPR) and transverse surface plasmon resonance (TSPR), because of the anisotropic geometry. Based on Maxwell’s equations, the multiple-multipole (MMP) method was used to investigate the electromagnetic field of a plane wave illuminating these nanorods. The research is mainly divided into two parts. For the optical properties, we focused on the average optical effects of a plane wave illuminating a GNRs. The average scattering, absorption and extinction efficiency, and the average depolarization ratio of a randomly oriented GNR were analyzed. A simple model, three-principal-axes method, was also used to calculate these efficiencies. The result showed that the simple model can only be useful for slim GNRs which have only the first SPR mode. For the mechanical responses, Maxwell stress tensor is used to investigate optical force and optical torque produced by the electromagnetic field on GNRs and SNRs. Simulation results show that there are two alignment modes of GNRs and SNRs. One is perpendicular mode; the range is between of LSPR and TSPR. The other one is parallel mode for those wavelengths longer than LSPR. The turning point between the two modes is at LSPR with a null optical torque. The two different modes can help us to explain why under the irradiance of the same NIR laser the alignments of nanorod and nanowire are different. For the former, the long axis is aligned to parallel to the polarization of a linearly polarized light. In contrast, the latter is aligned to perpendicular to the light polarization. This is because that the LSPR of GNR or SNR is red-shifted as the aspect ratio increases.